The present invention concerns a method and an instrument for the fast measurement of mass spectra from sample molecules, a so-called "scanning procedure", using a QUISTOR mass spectrometer.
This special type of mass spectrometer can store ions of different mass-to-charge ratios simultaneously in its radio-frequency hyperbolic three-dimensional quadrupole field. It has been called "QUISTOR" ("Quadrupole Ion STORe") or "ion trap" in the literature. See also our U.S. Pat. No. 4,882,484 issued Nov. 21, 1989 concerning certain of these Quistor applications.
The QUISTOR usually consists of a toroidal ring electrode and two end cap electrodes. A high RF voltage with amplitude V.sub.stor and frequency f.sub.stor is applied between the ring electrode and the two end caps, eventually superimposed by a DC voltage.
The hyperbolic RF field yields, integrated over a full RF cycle, a resulting force on the ions directed towards the center. This central force field forms, integrated over time, an oscillator for the ions. The resulting oscillations are called the "secular" oscillations of the ions within the QUISTOR field. The secular movements are superimposed by the oscillation impregnated by the RF storage field.
Cylindrical coordinates are used to describe the QUISTOR. The direction from the center towards the saddle line of the ring electrode is called the r direction or r plane. The z direction is defined to be normal to the r plane, and located in the axis of the device.
Up to now, the exact mathematical description, in an explicit and finite form, of the movements of ions in a QUISTOR field is only possible for the special case of independent secular movements in the r and z directions. The solution of the corresponding "Mathieu"'s differential equations results in a QUISTOR of fixed design with an angle of z/r=1/1.414 (1.414=square root of 2) of the double-cone which is asymptotic to the hyperbolic field. In this case, the central force is exactly proportional to the distance from the center, and exactly directed towards the center. This defines a harmonic oscillator, and the resulting secular movements are exactly harmonic oscillations.
In this special case of an "harmonic QUISTOR", the secular oscillations can be calculated. The frequencies are usually plotted as "beta" lines in a so-called "a/q" diagram, where "a" is proportional to the DC voltage between ring and end electrodes, and "q" is proportional to the RF voltage. The beta lines describe exactly the secular frequencies in the r and z directions: EQU f.sub.sec,r =beta.sub.r *f.sub.stor /2; EQU f.sub.sec,z =beta.sub.z *f.sub.stor /2.